As electronic circuits are becoming smaller and more integrated, an increasing number of portable products are becoming available. Portable computers and cellular telephones, for instance, are new devices that are growing in popularity. These devices rely upon batteries for mobility. Typically, it is more cost effective to operate such devices with a rechargeable battery pack. While a rechargeable battery may store less energy than does a single use battery, rechargeable batteries can be recharged hundreds of times, thereby providing a less expensive long term solution.
Just as portable computer and cellular phone technology advances, so does the technology associated with rechargeable battery packs. Circuits that measure battery energy storage, known as fuel gauge circuits, are becoming common place. There is also a heightened awareness of safety, and safety circuits are therefore being integrated into battery packs. The safety circuits are especially applicable to lithium based batteries, as strict voltage and current limits must be observed to reduce any potential safety hazards.
Designers of battery packs are becoming concerned with the current drain demanded by the complex circuitry contained within a battery pack. Current drawn within a battery pack reduces the energy available to the host electronic device. An especially pressing concern is the amount of current drain that occurs between the time a battery is manufactured and the time when it is first charged by a consumer. During this time, which could be months, the circuitry in the battery pack is generally consuming energy. This energy consumption causes battery discharge. If the charge level decreases too much, a condition known as over-discharge, the life cycle capacity of the battery will be reduced. At extreme over-discharge, the battery pack will be damaged and will then not store energy. This is particularly true in lithium based battery packs.
There are two obvious solutions to this problem of battery discharge after manufacture and before customer charge. First, the manufacturer can charge the battery to maximum capacity prior to warehouse storage and subsequent shipment. This is expensive, however, as it requires time, equipment, and personnel.
Second, the designer of the battery pack can use low power dissipation devices. However, this is also expensive, as it generally requires custom parts designed specifically for batteries. Such parts often ship in low volumes and are therefore expensive.
There is therefore a need for a means to address the problem of battery current drain between the time of manufacture and the time of delivery to an end customer.